Added support for EXT_frag_depth

This change also required that support be added for associating built-in
variables with an extension, similar to how functions could be associated with
extensions previously.

R=alokp@chromium.org

Review URL: https://codereview.appspot.com/9827044

git-svn-id: http://angleproject.googlecode.com/svn/trunk@2248 736b8ea6-26fd-11df-bfd4-992fa37f6226
Regenerated glslang_tab.cpp to fix compile errors

The version committed in Issue 9827044 was generated in a bad environment and doesn't compile againts top-of-tree. This version does.

BUG=429
R=alokp@chromium.org

Review URL: https://codereview.appspot.com/9738044

git-svn-id: http://angleproject.googlecode.com/svn/trunk@2249 736b8ea6-26fd-11df-bfd4-992fa37f6226